Method and system for improving plug-and-play performance of network construction type converter in direct-current micro-grid

Abstract

The invention belongs to the technical field of direct-current micro-grid control, and particularly discloses a method and system for improving the plug-and-play performance of a network construction type converter in a direct-current micro-grid. According to the method, an additional controller Q is additionally arranged on the basis of an original PI double-closed-loop controller of each grid-forming type DC-DC buck converter in the DC micro-grid, and a multi-converter interconnection control system state space model comprising the additional controller is established. The stability condition of the system is decomposed through the Lyapunov stability theory, a Lyapunov matrix and gain parameters of an additional controller Q are designed, and the overall stability of the system is ensured. Meanwhile, the gain of the additional controller Q is optimized, the response performance of the distributed power supply is improved, and the amplitude of the control signal is limited. The invention further discloses a corresponding control system, a computing unit and a computer readable storage medium.

Description

Technical Field

[0001] The present invention belongs to the technical field of DC microgrid control, and specifically discloses a method and system for improving the plug-and-play performance of grid-forming converters in a DC microgrid. Background Art

[0002] With the popularization of the application of DC power generation and consumption equipment such as photovoltaic power generation, energy storage, and DC charging, compared with AC microgrids, DC microgrids are a more efficient form of DC power generation and consumption operation management due to their high conversion efficiency, simpler control methods, and no reactive power distribution problems. However, in a DC microgrid, due to reasons such as system expansion and equipment failures, distributed generation equipment may be connected to or disconnected from the DC microgrid, causing obvious changes in the system structure, resulting in problems such as the original control strategies of the DC-DC buck converters of each power generation device may no longer be applicable, the overall stability or stability margin of the system decreases, and the overall control performance of the system deteriorates. It is difficult for the DC-DC buck converters in the DC microgrid to achieve plug-and-play in the DC microgrid.

[0003] To achieve plug-and-play control of the buck converters of distributed power sources in a DC microgrid, there are currently different control design methods, mainly including the control design method of distributed devices based on passive characteristics and the controller design method based on robust theory.

[0004] a) The design method based on dissipation theory is a decentralized design method for controllers, which can independently design the control strategies of each buck converter. When the control characteristics of each buck converter are passive to the outside, when multiple buck converters are connected to the microgrid, the overall stability of the system can still be guaranteed. The advantage of this method is that it can independently design the control strategies of each distributed power source converter without relying on line and other distributed power source parameters. However, the disadvantage is that this method is relatively conservative in design, the overall control performance of the system is not high, and this method requires a re-design or modification of the direct control of the buck converter, which may be difficult to modify for some existing equipment.

[0005] b) The design method based on robust theory is a method that utilizes Lyapunov stability and robust control theory. On the basis of considering uncertainties such as the connection and disconnection of distributed power sources in the system, it designs the system stability constraint conditions under all possible situations, and solves the buck converter controller through LMI (Linear Matrix Inequality). This enables the DC microgrid to maintain system stability and effective operation even after some distributed devices are connected or disconnected, realizing the plug-and-play ability of the buck converter. The advantage of this method is its high robustness to system uncertainties; the disadvantage is that it needs to consider various uncertainties caused by system changes, which may lead to a relatively complex design. And because it needs to meet the stability under various system uncertainties, the system design may be relatively conservative and even the controller parameters may have no solution. At the same time, this method still requires the re-design and modification of the controller, making it difficult to apply to existing devices.

[0006] Therefore, the main problems of the current design methods for the plug-and-play controller of the grid-forming converter in the DC microgrid are summarized as follows:

[0007] 1. The current two existing controller design methods based on passive characteristics and robust stability are relatively conservative, with insufficient control performance of the converter, and even the controller design may have no solution;

[0008] 2. The current methods all require the re-design of the device control strategy, and it is difficult to apply to the existing Buck converters when the manufacturer does not open the permission to modify the controller. Summary of the Invention

[0009] In view of the above problems, the present invention discloses a method and system for improving the plug-and-play performance of the grid-forming converter in the DC microgrid; the method of the present invention in the DC microgrid, without changing the original control strategy of the buck converter, designs an additional control algorithm on the basis of the existing controller of the buck converter to improve the overall control performance of the microgrid, and ensures that the microgrid can still remain stable after some distributed power sources in the DC microgrid are connected or disconnected, thereby realizing the plug-and-play ability of the buck converter in the DC microgrid.

[0010] The object of the present invention is achieved by the following technical solutions.

[0011] A method for improving the plug-and-play performance of the grid-forming converter in the DC microgrid, comprising the following steps:

[0012] (1) On the basis of the original PI double closed-loop controller of each grid-forming DC-DC buck converter in the DC microgrid, add an additional controller Q;

[0013] (2) Establish a state - space model of the multi - converter interconnected control system including the additional controller Q, and the model describes the system dynamic characteristics by decomposing into local terms and interconnected coupling terms;

[0014] (3) Based on the Lyapunov stability theory, decompose the system stability conditions into local - term stability conditions and interconnected - coupling - term stability conditions, and ensure the overall system stability by designing the Lyapunov matrix and the gain parameters of the additional controller Q;

[0015] (4) Optimize the gain of the additional controller Q to improve the response performance of the distributed power source under the condition of meeting the stability constraint conditions, and limit the amplitude of the control signal to avoid exceeding the device capacity limit.

[0016] Further, for the above - mentioned method of improving the plug - and - play performance of the grid - forming converter in the DC micro - grid, the output signal of the additional controller Q is superimposed on the output signal of the original PI controller to form the total control signal.

[0017] Further, for the above - mentioned method of improving the plug - and - play performance of the grid - forming converter in the DC micro - grid, the interconnected - coupling - term stability condition is realized by constructing a Laplacian matrix that satisfies semi - negativity.

[0018] Further, for the above - mentioned method of improving the plug - and - play performance of the grid - forming converter in the DC micro - grid, the optimization objective of the additional controller Q is to minimize the real part of the system poles, and the optimal gain value is solved by a numerical optimization toolbox.

[0019] The present invention also discloses a control system for improving the plug - and - play performance of the grid - forming converter in the DC micro - grid, which is characterized by including:

[0020] (1) Multiple grid - forming DC - DC buck converters: Each converter is configured with a PI double - closed - loop controller;

[0021] (2) An additional controller module: Connected in parallel with the PI controllers of each converter, used to generate a compensation control signal;

[0022] (3) A modeling module: Used to construct a state - space model of the multi - converter interconnected control system including the additional controller;

[0023] (4) A stability analysis module: Based on the Lyapunov theory, verify the system stability and design the gain parameters;

[0024] (5) An optimization module: Used to optimize the gain value of the additional controller under the condition of meeting the stability constraint conditions.

[0025] Further, for the above - mentioned system, the (2) additional controller module adopts a decentralized design and does not need to rely on other converter parameters or line parameters.

[0026] Further, in the above system, the (5) optimization module ensures that the control signal does not exceed the device capacity limit by restricting the gain amplitude.

[0027] Further, in the above system, the system supports the plug-and-play of distributed power sources and automatically maintains the stable operation of the microgrid after the power source is connected or disconnected.

[0028] The present invention discloses a computing unit for executing the above method, including:

[0029] a modeling unit: for constructing the state space model of the multi-converter interconnected control system;

[0030] b stability determination unit: decomposing the stability conditions and designing matrix parameters based on the Lyapunov stability theory;

[0031] c optimization unit: solving the optimal gain value of the additional controller Q through a numerical optimization toolkit;

[0032] d control signal generation unit: outputting the optimized gain value to the additional controller module.

[0033] The present invention also discloses a computer-readable storage medium storing a computer program, and when the computer program is executed by a processor, the steps of the above method for improving the plug-and-play performance of the network-forming converter in a DC microgrid are implemented.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The present invention discloses a control method and system for improving the plug-and-play performance of the network-forming converter in a DC microgrid.

[0036] Based on the Lyapunov stability analysis, the present invention can realize the decentralized design of distributed converters without the need for line parameters between nodes and other converter parameters.

[0037] The present invention can ensure the stability of the DC microgrid and improve the plug-and-play performance and control response performance of the device under the condition of connecting and disconnecting distributed devices without modifying the existing control strategy of the buck converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 Schematic diagram of the structure of a multi-bus DC microgrid with multiple distributed micro-sources;

[0039] Figure 2 Schematic diagram of the principle for improving the plug-and-play control performance of the buck converter in a DC microgrid based on an additional controller. DETAILED DESCRIPTION OF THE INVENTION

[0040] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the following will clearly and completely describe the technical solutions in the embodiments of this application in conjunction with the accompanying drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, rather than all of them. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of this application.

[0041] Embodiment 1

[0042] This embodiment discloses a method for improving the plug-and-play performance of grid-forming converters in a DC microgrid, including the following steps:

[0043] (1) Based on the original PI double-loop controller of each grid-forming DC-DC buck converter in the DC microgrid, an additional controller Q is added.

[0044] (2) Establish a state-space model of the multi-converter interconnected control system including the additional controller Q, and the model describes the system dynamic characteristics by decomposing into local terms and interconnected coupling terms.

[0045] (3) Based on the Lyapunov stability theory, decompose the system stability conditions into local-term stability conditions and interconnected coupling-term stability conditions, and ensure the overall system stability by designing the Lyapunov matrix and the gain parameters of the additional controller Q.

[0046] (4) Optimize the gain of the additional controller Q, improve the response performance of distributed power sources under the condition of meeting the stability constraint conditions, and limit the amplitude of the control signal to avoid exceeding the device capacity limit.

[0047] The output signal of the additional controller Q is superimposed on the output signal of the original PI controller to form a total control signal.

[0048] The stability condition of the interconnected coupling term is achieved by constructing a Laplace matrix that satisfies semi-negative definiteness for the system Lypunove stability judgment matrix.

[0049] The optimization objective of the additional controller Q is to minimize the real part of the system poles, and the optimal gain value is solved through a numerical optimization toolbox.

[0050] As Figure 1 is a schematic diagram of the multi-bus DC microgrid structure with multiple distributed micro-sources of the present invention, Figure 2 showing the principle of improving the plug-and-play performance of the DC microgrid buck converter. As Figure 2 shown, based on the PI double-loop controllers of each buck converter in the microgrid, an additional controller Q is designed, and the additional controller Q outputs an additional control signal uQ , realize the plug-and-play ability of each converter in the microgrid, and while ensuring the stability of the microgrid, improve the response ability of each converter at the same time.

[0051] Embodiment 2

[0052] This embodiment discloses an additional controller Q decentralized design method for a DC microgrid buck converter:

[0053] 1) Establish the state space model of the DC microgrid DC-DC buck converter interconnection system and the buck converter's own PI controller:

[0054] 1.1) State space model of the DC microgrid interconnection system with DC-DC buck converters:

[0055]

[0056] Denote the above equation as

[0057]

[0058] y m = x m (1b)

[0059] In the formula, the state variables of converter m V Cm , i Lm are the LC filter capacitor voltage and inductor current of converter m, L m , C m , R Lm , R mn , are the LC filter inductor, filter capacitor, inductor parasitic resistance, and the resistance of the connection line between busbars mn, respectively, R mn .

[0060]

[0061] The total control signal u of the converter m = u km + u Qm , u km is the output control signal of the original PI controller, and u Qm is the compensation control signal of the additional controller. d m is the load current disturbance i load,m , P load,m is the constant power load, after linearization at the operating voltage point

[0062] 1.2) State space model of the double closed-loop PI controller:

[0063]

[0064] That is

[0065]

[0066] u km = C km x km + D km y m + F km V refm (2b)

[0067] In the formula: the input signal of the PI double closed-loop controller is the converter output feedback y m , and the output control signal of the PI controller is u km , k p1 , k i1 are the proportional and integral gains of the voltage-loop PI controller, and k p2 , k i2 are the proportional and integral gains of the current-loop PI controller.

[0068]

[0069] C km = [k p2 k i1 k i2 , D km = [-k p2 k p1 -k p2 , F km = k p2 k p1

[0070] 1.3) Substituting equation (2b) into equation (1b) gives the state space of the interconnected system with a PI controller:

[0071]

[0072] In the formula

[0073]

[0074] 1.4) The performance improvement additional controller Q is a constant gain vector, that is

[0075] Q m = [q1, q2, q3, q4] (4)

[0076] Q m The output signal of the controller is

[0077] u Qm,1×1 = Q m,1×4 xPI+Gm,4×1

[0078] 1.5) Establish the state - space model of the multi - converter interconnected control system including the additional controller Q for performance improvement. Substitute the control output signal of Q into the controlled object. The state - space of the control system of the m - th converter is as follows:

[0079]

[0080] In the formula

[0081]

[0082] Denote Equation (6.1) as

[0083]

[0084] In the formula

[0085]

[0086] 1.6) The overall state - space model of the multi - converter interconnected control system including the original PI controller and Q is:

[0087]

[0088] Denote the above global state - space model as

[0089]

[0090] In the formula

[0091]

[0092] 2) For the system state - space Equation (7.2), decompose the block - matrix A g on the diagonal of A gmm as:

[0093]

[0094] 3) The overall Lyapunov stability condition of the system Equation (7.2) is:

[0095]

[0096] In the formula:

[0097]

[0098] Then there is

[0099]

[0100] The entire system matrix A g can be decomposed into a diagonal matrix A diag and a coupling matrix A coup : A g = A diag + A coup . Substituting into Equation (9.1), the system stability condition can be transformed into:

[0101] (A diag + A coup ) T P g + P g (A diag + A coup ) < 0 (10a)

[0102] can be transformed into

[0103] ((A diag ) T P g + P g A diag ) + ((A coup ) T P g + P g A coup ) < 0 (10b)

[0104] Finally, the overall control system stability determination inequality (10b) can be decomposed into:

[0105] Diagonal term stability condition: (A diag ) T P g + P g A diag < 0 (11a)

[0106] Interconnection coupling term stability condition: (A coup ) T P g + P g A coup ≤ 0 (11b)

[0107] When the above two conditional equations (11a) and (11b) hold simultaneously, it can ensure that Equation (9.1) holds and the overall microgrid system is stable.

[0108] 4) Design the matrix P to ensure that the interconnection coupling term stability conditional equation (11b) is a negative semi - definite matrix:

[0109] The interconnection coupling term stability conditional equation (11b) expands to:

[0110]

[0111] Let the system Lyapunov matrix \(P\) m , \(P\) n , \(P\) k satisfy the following conditions:

[0112]

[0113] Then the stability inequality of the coupling term of the entire system in Equation (11b) is

[0114]

[0115] The above matrix is a symmetric matrix, and the sum of each row and the sum of each column are both 0, the diagonal elements are negative, and other elements are all ≥ 0. Therefore, the above coupling term Lyapunov stability judgment matrix is the Laplacian matrix \(L\), which is a negative semi-definite matrix.

[0116] 5) Then design each \(Q\) to make the left matrix of Equation (11a) negative definite or negative semi-definite:

[0117]

[0118] That is, only need to satisfy

[0119]

[0120] where:

[0121]

[0122] Satisfy within the constant power load range \(P\) load = [0, \(P\) load,max , the following LMI matrix is negative definite:

[0123]

[0124] 6) At the same time, in order to improve the response ability of the system's distributed power supply, under the condition of ensuring the overall stability of the system, make the poles \(\lambda\) of each distributed power supply move as far to the left as possible:

[0125] Finally, for \(m = 1, 2,..., N\), \(Q\) m The optimization goal is:

[0126]

[0127] The equality and LMI inequality constraints of the matrix \(P\) are

[0128]

[0129] And in order to avoid the control signal amplitude being too large and exceeding the device capacity limit, limit the \(Q\) gain size:

[0130] ||Q m,1×4 ||2 ≤ q max (17.3)

[0131] Finally, use a numerical optimization toolkit such as Yalmip to numerically optimize Equation (16). On the basis of satisfying Equations (17.1), (17.2), and (17.3), find the Q values of each converter that minimize Equation (16).

[0132] Embodiment 3

[0133] This embodiment discloses a control system for improving the plug-and-play performance of a grid-forming converter in a DC microgrid, which is characterized by including:

[0134] (1) Multiple grid-forming DC-DC buck converters: Each converter is configured with a PI double closed-loop controller;

[0135] (2) An additional controller module: Connected in parallel with the PI controller of each converter, used to generate a compensation control signal;

[0136] (3) A modeling module: Used to construct a state-space model of a multi-converter interconnected control system including the additional controller;

[0137] (4) A stability analysis module: Verify the system stability based on Lyapunov theory and design gain parameters;

[0138] (5) An optimization module: Used to optimize the gain value of the additional controller under the condition of satisfying the stability constraint conditions.

[0139] The said (2) additional controller module adopts a decentralized design and does not need to rely on other converter parameters or line parameters.

[0140] The said (5) optimization module ensures that the control signal does not exceed the device capacity limit by restricting the gain amplitude.

[0141] The said system supports the plug-and-play of distributed power sources and automatically maintains the stable operation of the microgrid after the power source is connected or disconnected.

[0142] Embodiment 4

[0143] This embodiment discloses a computing unit for executing the method described in Embodiment 1 or 2, including:

[0144] a modeling unit: Used to construct the state-space model of the multi-converter interconnected control system;

[0145] b stability determination unit: Decompose the stability conditions based on Lyapunov stability theory and design matrix parameters;

[0146] c Optimization unit: Solve the optimal gain value of the additional controller Q through a numerical optimization toolkit;

[0147] d Control signal generation unit: Output the optimized gain value to the additional controller module.

[0148] As can be seen from the above embodiments, based on Lyapunov stability analysis, the present invention realizes the decentralized design of the distributed converter without relying on other converter parameters or line parameters. Without modifying the existing control strategy of the buck converter, the stability of the DC microgrid after the access or disconnection of distributed devices is ensured, and the plug-and-play performance and control response performance of the devices are improved. At the same time, by restricting the gain amplitude, the problem of the control signal exceeding the device capacity limit is avoided.

[0149] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Therefore, based on the innovative concept of the present invention, any changes and modifications made to the embodiments described herein, or equivalent structural or equivalent process transformations made using the content of the present invention's specification, directly or indirectly applying the above technical solutions to other related technical fields, are all included in the protection scope of the present invention patent.

Claims

1. A method for improving the plug-and-play performance of grid-forming converters in a DC microgrid, characterized in that, It includes the following steps: (1) On the basis of the original PI double closed-loop controller of each grid-forming DC-DC buck converter in the DC microgrid, an additional controller Q is added; (2) Establish a state space model of the multi-converter interconnected control system including the additional controller Q, and the model describes the system dynamic characteristics by decomposing into local terms and interconnected coupling terms; (3) Based on the Lyapunov stability theory, decompose the system stability conditions into local term stability conditions and interconnected coupling term stability conditions, and ensure the overall stability of the system by designing the Lyapunov matrix and the gain parameters of the additional controller Q; (4) Optimize the gain of the additional controller Q, improve the response performance of the distributed power supply under the condition of meeting the stability constraint conditions, and limit the amplitude of the control signal to avoid exceeding the equipment capacity limit.

2. The method according to claim 1, characterized in that, The output signal of the additional controller Q is superimposed on the output signal of the original PI controller to form a total control signal.

3. The method according to claim 1, wherein The stability condition of the interconnected coupling term is realized by constructing a Laplace matrix that satisfies semi-negative definiteness.

4. The method according to claim 1, characterized in that, The optimization objective of the additional controller Q is to minimize the real part of the system poles, and the optimal gain value is solved by a numerical optimization toolbox.

5. A control system for improving the plug-and-play performance of network-forming converters in a DC microgrid, characterized in that, It includes: (1) Multiple grid-forming DC-DC buck converters: Each converter is configured with a PI double closed-loop controller; (2) An additional controller module: connected in parallel with the PI controllers of each converter, used to generate a compensation control signal; (3) A modeling module: used to construct a state space model of the multi-converter interconnected control system including the additional controller; (4) A stability analysis module: verify the system stability based on the Lyapunov theory and design the gain parameters; (5) An optimization module: used to optimize the gain value of the additional controller under the condition of meeting the stability constraint conditions.

6. The system according to claim 5, wherein The (2) additional controller module adopts a decentralized design and does not need to rely on other converter parameters or line parameters.

7. The system according to claim 5, wherein The (5) optimization module ensures that the control signal does not exceed the equipment capacity limit by limiting the gain amplitude.

8. The system according to claim 5, wherein The system supports the plug-and-play of distributed power supplies and automatically maintains the stable operation of the microgrid after the power supply is connected or disconnected.

9. A computing unit, characterized in that, For implementing the method according to any one of claims 1-4, it includes: a modeling unit: used to construct the state space model of the multi-converter interconnected control system; b stability determination unit: decompose the stability conditions based on the Lyapunov stability theory and design the matrix parameters; c optimization unit: solve the optimal gain value of the additional controller Q through a numerical optimization toolbox; d control signal generation unit: output the optimized gain value to the additional controller module.

10. A computer-readable storage medium storing a computer program, characterized in that, When the computer program is executed by a processor, it realizes the method steps according to any one of claims 1-4.

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